Annually, at the time of levying county taxes, the board of supervisors shall levy a tax to be known as the “_______ (name of district) boulevard district tax,” sufficient to raise the amount estimated by the commission to be necessary for the purposes of the district for the ensuing fiscal year. The board of supervisors shall determine the rate of the tax by deducting 15 per cent for anticipated delinquencies from the total assessed value of the real property within the district, as it appears on the assessment roll of the county, and dividing the sum required to be raised, by the remainder of the total assessed value.